Direct memory access (DMA) signal processing is well-known in information transfer systems for storing in a random access memory directly accessible by a processor, information including data received from one or more sources of signal transmissions. The purpose of the DMA circuitry is to provide ready memory access by the processor to the received data with a minimum of processor time. The DMA circuitry receives the data from one or more sources and stores the data at addressable locations in the memory which are designated by the processor.
In a typical information transfer system, such as a data communication channel where a data link is used to transfer data between a host processor and a remote data center location, a microprocessor-controlled direct memory access interface unit temporarily stores blocks of data received from both the host processor and the data link in a random access memory for subsequent transmission to the other unit after a complete block of data has been received. This is normally done to interface two units that transmit data at different bit rates, have different levels of data protocol, etc. Typically, the interface unit includes a peripheral unit having a program-controlled microprocessor and an associated program memory to transfer data and a peripheral unit controller to control the transfer of data between the host processor and data link.
However, in the prior art, neither the host processor nor the data link can access the peripheral program memory to modify, alter, or load the microprocessor program. A portion of the program may be stored in the random access memory, but at least one portion of the program which contains a recovery or bootstrap routine must be stored in a permanent type memory such as a read only memory.
In addition to the cost of two types of memories, the permanent memory causes another problem. Every time the microprocessor program is changed, for whatever reason, the permanent memory needs to be replaced or physically modified. When a large number of interface units are utilized such as, for example, with telecommunication switching systems transmitting billing information to a remote billing data center, the cost of maintenance personnel to replace or modify the permanent program memory in a large number of switching systems across the country is enormous. Hence, a direct memory access interface unit that is directly accessible by a host processor to load a microprocessor program in a random access memory is highly desirable.